Impurity effect in electron-doped high-T _c superconductors

Abstract

The quasiparticle states around a nonmagnetic impurity in electron-doped high-T c superconductors are studied systematically based on the Bogoliubov-de Gennes equations. In the antiferromagnetic state, one in-gap impurity resonance state is revealed. As the impurity scattering potential increases, the resonance peak shifts to the gap edge and finally disappears for a strong impurity. The antiferromagnetic order and superconducting order coexist when the doping density increases. In this coexisting state, the in-gap resonance peaks are rather robust and appear in pairs that are lying symmetric with the Fermi energy. The peak positions and intensities strongly depend on the impurity potentials and the next-nearest-neighbour hopping constants. For a rather strong impurity, the resonance peaks shift to near the gap edge. When further increasing the doping density, the system is in the pure superconducting state. The resonance peaks still appear in pairs, with the peak intensities being weaker compared to those in the coexisting state. The two resonance peaks may occasionally merge into one zero-energy peak for both the coexisting state and the pure superconducting state. The spin-resolved LDOS are also investigated and may be used to detect possible antiferromagnetic order.